Bioinspired design of Na-ion conduction channels in covalent organic frameworks for quasi-solid-state sodium batteries

Solid polymer electrolytes are considered among the most promising candidates for developing practical solid-state sodium batteries. However, moderate ionic conductivity and narrow electrochemical windows hinder their further application. Herein, inspired by the Na+/K+ conduction in biological membranes, we report a (–COO–)-modified covalent organic framework (COF) as a Na-ion quasi-solid-state electrolyte with sub-nanometre-sized Na+ transport zones (6.7–11.6 Å) created by adjacent –COO– groups and COF inwalls. The quasi-solid-state electrolyte enables selective Na+ transport along specific areas that are electronegative with sub-nanometre dimensions, resulting in a Na+ conductivity of 1.30×10–4 S cm–1 and oxidative stability of up to 5.32 V (versus Na+/Na) at 25 ± 1 °C. Testing the quasi-solid-state electrolyte in Na||Na3V2(PO4)3 coin cell configuration demonstrates fast reaction dynamics, low polarization voltages, and a stable cycling performance over 1000 cycles at 60 mA g–1 and 25 ± 1 °C with a 0.0048% capacity decay per cycle and a final discharge capacity of 83.5 mAh g−1.

(7) Moreover, pouch cells should be used in demonstrating the commercialization prospects of the quasi-solid electrolyte. Unfortunately, it also cannot be found in this manuscript. The evaluation is suggested to be provided.
(8) The thickness of the membrane is too thick (400 μm), which adversely affects the energy density of the batteries. The decrease in thickness should be considered.
(9) How about the performances without adding any PC solvent, and in this case the intrinsically ionic conductivity can be evaluated?
(10) For the specific surface area of TPDBD-CNa-NaTFSI and TPBD-NaTFSI are 38 and 28 m2 g-1, respectively. And the weight loss of TPDBD-Can and TPDBD are shown as 44% and 46%, respectively. From the curves of Fig. S14, this could be due to measurement error.
(11) For studying the chemical composition of the solid electrolyte interface (SEI), this manuscript only carried out the XPS experiments of surface. But it is far from enough for the SEI analysis. For example, the XPS depth profiling analysis or the time-of-flight secondary ion mass spectrometry depth profiling analysis should be considered.
(12) For Table S2, the thickness of electrolyte should be also included for better comparison.
(13) Other minor issues should be taken seriously: a) The abbreviation of Fourier transform infrared spectroscopy should be unify, such as FT-IR in line 215 while FTIR in 162. b) In line 192, the "a" in "-NaTFSI" should be in "Times New Roman" style.
c) The format of references should be uniform, such as title capitalization and journal abbreviations.
Reviewer #3 (Remarks to the Author): In this article, authors fabricated quasi-solid polymer electrolyte using TPBD-NaTFSI and TPDBD-CNa-NaTFSI as the support substrate by absorbing PC and FEC solvent. In this way, high ion conductivity and high oxidative potential is achieved.
The strength of this work is the biomimetic inspired synthesis of a new type of COF materials as the host for Na ions conduction. the polymer electrolytes. Authors repeatedly emphasizes that the -COO-functional group anchored on the COF wall promotes the decomposition of NaTFSI and boosts the Na ion transport. The calculation was conducted based on this assumption. However, this assumption is questionable. Another more possible reason to the enhanced ion conductivity is that the functionalized COF has a stronger capability in retaining the solvent solution. The observed effect is not limited to such COF. Instead, similar effect could also be realized from materials that have a high specific surface area, porous structure and solvent-capturing functional groups, for example, molecular sieves and OH-functionalized cellulose. The conclusion will not be convincing unless authors can prove that the Na ions conduct in the COF host much faster than in solution, and/or the effect of PC solvent to Na ion conduction has been explicitly clarified.
Other minor issues: 1) it is not meaningful to test the thermal stability of pure TPBD-NaTFSI and TPDBD-CNa-NaTFSI; instead, it is more relevant for battery practice to test the thermal stability of their mixture with solvent.
2) the LSV test shows a wide electrochemical stability window of 5.32 V. This is good for the electrolyte membrane itself but it does not mean the full-cell battery can deliver such high voltage. The actual output voltage as well as power performance of a Na-ion battery based on quasi-solid polymer electrolytes is more significantly affected by the interface property. So it could be misleading to claim a high-performance full cell based on high-performance of the electrolyte membrane. This manuscript reports a -COOmodified covalent organic framework (COF) as Na + -5 conducting quasi-solid-state electrolyte (QSSE) for Na metal batteries. It is of 6 significance to come up with solid or quasi-solid-state electrolytes with high 7 conductivity and wide electrochemical windows for advanced Na batteries, and the 8 work deals with this important topic. Through a series of experimental, DFT,and MD 9 analyses, the authors demonstrated that the carboxylic acid groups (-COO -) anchored 10 on the COF inwalls construct sub-nanochannels that can dissociate NaTFSI and thus 11 transport Na ions while enhancing the oxidative stability of the polymer electrolytes. 12 The proposed electrolyte exhibited high ionic conductivity and Na + transference 13 number and facilitated uniform Na plating/stripping. The paper would be interesting to 14 researchers active in the area of solid-state Na batteries. However, the manuscript 15 needs improvement as commented below: 16 Reply: Thanks for your thoughtful comments. We have tried our best to revise the 17 manuscript according to your valuable comments and suggestions.  to the reviewer's suggestion, we provide a more detailed description of the "Pendulum" 23 transport mechanism in the revised manuscript (Please see Highlights in Page 11 of the 24 revised manuscript). The relevant discussion is as follows: due to the electronegative 25 modification of -COOgroups, Na + is preferentially adsorbed in the sub-nanometer-26 sized zones (constructed by the adjacent -COOgroups and COFs inwalls), while TFSI -27 is repulsed at the center of COF channels. Such sub-nanometer-sized zones are like the 28 biomimetic Na + transfer channels, which can effectively promote the Na + transport 29 efficiency directionally along the oxygen atoms. In fact, the jumping transfer mode of 30 Na + from one carboxylic/carbonyl position to the next unoccupied site under a given 31 voltage can be regarded as a kind of "Pendulum" Na + transport mechanism, yielding a 32 high Na + conduction performance. the band gap (Eg) of TPDBD is determined to be 2.17 eV, larger than that of TPBD

52
(1.90 eV), further implying the better electron-insulating property of TPDBD, which is 53 more reliable for the solid-state electrolyte. FTIR spectrum of TPDBD-CNa-QSSE 54 shows stronger propylene carbonate (PC)-Na + interactions (720 and 1403 cm −1 ) and 55 concentrated TFSI − (787 cm −1 ) than TPBD-QSSE, demonstrating stronger electrolyte 56 aggregation effect (Figure R2a). Due to the high confinement of TPDBD-CNa, a tiny 57 amount of PC tends to be rapidly captured into the sub-nanometer channels of TPDBD-58 CNa-QSSE. As shown in Figure R2b, the TPDBD-CNa-QSSE and TPBD-QSSE are 59 more thermostable than pure PC and glass fiber separator filled with PC, making it 60 harder to oxidize and decompose the quasi-solid-state electrolyte, further verifying the 61 excellent interfacial contact and compatibility of PC confined in the QSSEs. Therefore, 62 the oxidative decomposition of the quasi-solid-state electrolyte requires a higher force, 63 which is another reason for the high voltage window of TPDBD-CNa-QSSE (Nat.     assembling NVP/C cathode, QSSE, and Na anode in an argon-filled glove box (H2O 88 and O2 <1 pmm). The cathodes were prepared by homogeneously blending NVP/C 89 (0.14 g), QSSEs, acetylene black, and polyvinylidene difluoride (PVDF) at a mass ratio 90 of 14:3:2:1 in N-Methyl-2-pyrrolidone (NMP), and then directly stirring for 3 h to get 91 a viscous solution. The resulting slurry was uniformly coated on a conductive carbon-92 coated Al foil and dried in a vacuum oven at 120 °C for 12 h. The NVP/C cathode was 93 cut into 12 mm in diameter for CR2032 assembly. The mass of specific current (mA 94 g −1 ) refers to the mass of active material (1.3-2.6 mg cm −2 ) in the cathode, and the non-95 electrochemical active materials was 0.7-1.4 mg cm −2 , and current collector was about 96 4.6 mg cm −2 . The volume of Na anode was 100 or 300 μm × 0.95 cm 2 . As for pouch 97 cell fabrications, the cathode was cut into a rectangle (3 × 4 cm 2 in size), and the total 98 mass is 25.6 mg. All the electrochemical tests were carried out at 25 ± 1 °C except the 99 especial declaration. 100 Also, we had investigated the electrochemical performances of TPDBD-CNa-QSSE 101 battery with high mass loading of active materials in the cathode (2.6 mg cm −2 ) and thin 102 Na anode (~100 μm). According to the editor's suggestion, the specific current (mA g −1 ) 103 values instead C-rate values when electrochemical energy storage tests are discussed. 104 1 C charging and discharging rate here was defined as 120 mA g −1 . As shown in Figure   105 R3, the NVP/C|TPDBD-CNa-QSSE|Na retains 63 mAh g −1 after 100 cycles at 60 mA 106 g −1 with a coulombic efficiency of 99.6%, and displays 99.7 mAh g −1 after 68 cycles at 107 12 mA g −1 with a coulombic efficiency of 99.6%. When the current densities are 12, 24, 108 60, and 120 mA g −1 , the specific capacities of NVP/C|TPDBD-CNa-QSSE|Na are 101.5, 109 106, 91.6 and 78.5 mAh g −1 , respectively. Even at a higher rate of 240 mA g −1 , the 110 battery still represents 54.5 mAh g −1 . When the current density recovers to 60 mA g −1 , 111 a specific capacity of 103.9 mAh g −1 can be obtained, revealing reversible 112 characteristics. Those results verify that the NVP/C|TPDBD-CNa-QSSE|Na full cell 113 with high mass loading of cathode shows acceptable rate performance and cycle 114 stability.

115
In this revision, we have supplemented this Figure R3 (Figure R5a), corresponding to the reversible transformation of V 3+ /V 4+ , exhibiting faster Na + reaction kinetic of NVP/C|TPDBD-CNa-QSSE|Na during Na 165 insertion/extraction process. As the current density increased to 120 mA g −1 , two pairs 166 of well-defined redox peaks appear in dQ/dV profiles, which can be attributed to the 167 dynamic hysteresis ( Figure R5c). However, due to the absence of biomimetic sub-168 nanochannels, the dQ/dV profile of NVP/C|TPBD-QSSE|Na exhibits multiple 169 platforms even at a small current density of 12 mA g −1 (Figure R5d Table   201 R1. Actually, the EO10-PFPE/Solupor composite electrolyte was used with the 202 assistance of commercial Solupor separator, and the symmetrical Na cell was performed   Table R1). 215 For comparison, we investigated the tripping-plating plots of Na|TPDBD-CNa-

216
QSSE|Na cell at high current densities of 0.02, 0.05, and 0.1 mA cm −2 without 217 additional pressure. As shown in Figure R6, the TPDBD-CNa-QSSE symmetrical cell 218 shows lower polarization and stable Na plating/stripping performance without any sign 219 of short circuits. Moreover, the symmetric battery of TPDBD-CNa-QSSE at 0.05 mA 220 cm −2 exhibits steady Na insertion/extraction processes for over 450 h without obvious 221 fluctuation of potential, indicating that there is good interfacial stability between QSSE 222 and Na metal anode. More significantly, the Na + conduction mechanism is deeply 223 analyzed through the biomimetic concept for the first time in our work, and ample 224 evidence has been carried out using the sodium ion full and soft pack battery to assess 225 the practical application of our QSSE, which may provide profound implications for 226 the promising energy storage fields.

227
In this revision, we have supplemented this Figure    to 74 m 2 g -1 and 0.30 cm 3 g -1 of TPDBD due to the internally cross-linked protic acid.

298
After Na-activation, the specific surface and pore volume of TPDBD-CNa re-increases 299 to 116 m 2 g -1 and 0.60 cm 3 g -1 by eliminating the protic acid cross-linked oligomers 300 inside the pores and layers. Compared with TPBD, the BET surface area and pore 301 volume of TPDBD exhibit obvious decrease due to the weak crystallinity induced by 302 the -COOmodification, that is consistent with the PXRD study ( Figure R7a). 303 In this revision, we have supplemented this Figure R8 (2022)).

331
Besides, compared with TPBD, the pore size of TPDBD is slightly larger due to the weak π-π stacking interactions. Therefore, in the follow-up simulation analysis, we 338 adopted the 1D channel model construction to analyze the ion conduction mechanism.

339
On the premise of maintaining well-aligned sub-nanochannels, the concept of 340 "preferentially adsorb Na + on the COF inwalls with TFSIrepulsed at the center of COF  Commun. 13, 1510 (2022)).

357
The Na-O bond length of NaTFSI in QSSEs becomes significantly longer, indicating 358 PC can effectively dissociate Na + and TFSI − (Figure R13b). The TGA on COF-based 359 membranes with solvent was performed ( Figure R13c). Due to the sub-nanometer QSSEs, thus effectively boosting the Na + conduction and wide working temperature.

365
Moreover, under electric field, along with Na + transport, some PC molecules are 366 confined into the sub-nanochannels due to the high interaction energy between 367 TPDBD-CNa and PC solvent, which can effectively prevent PC volatilization and 368 further promote Na + conduction (Figure R13d-e). It should be noted that the interaction 369 energy between TPDBD-CNa (-11 Kcal mol -1 )/ Na + (-1 Kcal mol -1 ) and PC were 370 calculated in TPDBD-CNa-NaTFSI/PC, and the stronger interaction energy between 371 PC and TPDBD-CNa framework promotes the adsorption of PC solvents at the sub-372 nanometer zones (Figure 13f), and a very small amount of PC may form solvated 373 cations, and it was more difficult for the solvated PC to be removed from TPDBD-CNa-

375
In this revision, we have supplemented this Figure R12 and Figure    Na + , and TFSI − of (NaTFSI).

415
(4) Fig. S18  AGG, we re-tested the Raman spectra many times. However, the specific peaks are 419 weak and it is really hard to divide due to the limited anion and cation aggregation 420 content. Even so, Figure R14 shows an obvious blue shift in TPBD-NaTFSI and    loadings of active materials (2.6 mg cm −2 ) and low N/P ratio (~100 μm Na anode).

496
According to the editor's suggestion, the specific current (mA g −1 ) values instead C-497 rate values when electrochemical energy storage tests are discussed. 1 C charging and 498 discharging rate here was defined as 120 mA g −1 . As shown in Figure R15,  (2016)). Actually, the reported anode-free sodium-ion batteries are 519 always assembled using hexafluoride phosphate (NaPF6) and sodium tetrafluoroborate 520 (NaBF4) in ether solvents to favor the high Na reversibility and low solubility of SEI.

521
Therefore, the electrochemical performance of the anode-free sodium-ion batteries is 522 closely related to the assembled components. In consideration of the promising 523 potential of anode-free Na + batteries especially anode-free solid-state Na + batteries, 524 great efforts will be paid for the deep studies of practical anode-free batteries in our 525 future works.

526
In this revision, we have supplemented this Figure R15        Reply: Thank you very much for the constructive question. As shown in Figure R20, 592 the Ti|TPDBD-CNa-NaTFSI|Ti without any PC solvent and addition pressure free was 593 performed to investigate the intrinsically ionic conductivity from 25 to 100 ºC. We find 594 that the blocking titanium symmetric cell cannot work properly below 100 °C due to 595 the weak contact between COF and titanium sheet. As the working temperature 596 increased to 100 °C (Figure R20c), the intrinsic Na + conductivity is about 4.49×10 -6 S 597 cm -1 . Therefore, the "Biomimetic Na + Channel" of TPDBD-CNa can perform effective 598 ion conduction even in solvent-free conditions.

599
In this revision, we have supplemented this Figure R20   Reply: Thanks for your important comment. In order to eliminate the measurement 614 error, we have re-tested the N2 adsorption and desorption of TPDBD-CNa-NaTFSI and 615 TPBD-NaTFSI ( Figure R21, Table R3), and the TGA of TPDBD-CNa and TPDBD 616 for three times (Figure R22). The surface area of each COF-NaTFSI was calculated 617 using the BET model with relatively good fitting. The pore size distribution was 618 calculated based on the N2 sorption isotherm by using the nonlocal density functional   Moreover, the C 1s, O 1s, F 1s, and Na 1s spectra were used to study the chemical 652 composition in different depths of the solid electrolyte interface (SEI, Figure R24). As   In this article, authors fabricated quasi-solid polymer electrolyte using TPBD-NaTFSI 764 and TPDBD-CNa-NaTFSI as the support substrate by absorbing PC and FEC solvent. 765 In this way, high ion conductivity and high oxidative potential is achieved. Firstly, we investigate the Na + conductivity of pure COF-NaTFSI without PC 788 addition and additional pressure at different temperatures. As shown in Figure R25, 789 due to the solid-solid contact between COF particles in the electrolyte membrane, 790 enormous grain boundary impedance can dominantly hinder the practical ionic transfer 791 across interfaces, resulting in a poor ionic conductivity of 4.49×10 -6 S cm -1 of pure 792 TPDBD. Even so, we can also find the advantage of biomimetic design in the Na + 793 transport because the pure COF of TPBD without Na + sub-nanochannels design shows 794 almost no Na + conductivity. The result is consistent with the verification in our 795 manuscript that the bionic design of Na + transport channels in COFs can boost the rapid 796 dissociation of Na + and TFSI -, and achieve selective rapid transport of Na + along the 797 localized zones. for instance furthest the performance with least solvent addition. As shown in Table   810 R5, although relative high ion conductivity can be obtained in the reported works, the 811 amount of PC addition was almost beyond 30 wt.%, which is very high and dominate 812 the practical ionic transport. In our work, the result indicates that the ionic conductivity 813 can reach as high as 1.30×10 -4 S cm -1 at 25 °C when the amount of solvent addition is 814 ~9 wt.%. However, without the biomimetic design, the ionic conductivity of TPBD is 815 only 9.06×10 −5 S cm -1 . The result means that the biomimetic design of COFs shows 816 great advantage in the efficient utilization of PC solvent. impedance between particles. In addition to the surficial wettability, along with Na + 828 transport under electric field, some PC molecules can be confined into the sub-829 nanochannels due to the strong interaction between TPDBD-CNa and PC solvent, 830 which can effectively prevent PC volatilization and further promote the Na + transport 831 in the biomimetic sub-nanochannels (Figure R27a-b). It should be noted that the 832 interaction energy between TPDBD-CNa (-11 Kcal mol -1 )/ Na + (-1 Kcal mol -1 ) and PC 833 were calculated in TPDBD-CNa-NaTFSI/PC, and the stronger interaction energy 834 between PC and TPDBD-CNa framework promotes the adsorption of PC solvents at 835 the sub-nanometer zones (Figure R27c), and a very small amount of PC may form 836 solvated cations, and it was more difficult for the solvated PC to be removed from 837 TPDBD-CNa-NaTFSI and then undergo oxidation. As shown in Figure R27d, 838 TPDBD-CNa-QSSE behaves stronger propylene carbonate (PC)-Na + interactions (720 839 and 1403 cm −1 ) and concentrated TFSI − (787 cm −1 ) than TPBD-QSSE, demonstrating 840 stronger electrolyte aggregation effect (Nat. Commun. 13, 1510 (2022)). DFT 841 calculations reveal that the Na-O bond length of NaTFSI becomes significantly longer, 842 and PC displays strong interaction with Na + , indicating PC can effectively dissociate 843 Na + and TFSI − (Figure R27e). TGA spectra of COF-based membranes with solvent 844 shows higher thermal stability than pure solvent and glass fiber/solvent (Figure R27f), 845 further verifying the strong confinement of PC in the COF interior. The high stability 846 of QSSEs is advantageous for the elimination of grain boundary impedance, thus 847 effectively boosting the Na + conductivity and wide working temperature.   Figure R26. here was defined as 120 mA g −1 . As shown in Figure R29, the NVP/C|TPDBD-CNa-